Technological advancements in computers, mobile telephones, digital cameras, entertainment devices, and other portable electronic devices, as well as battery powered or hybrid vehicles, have increased the demand for a power source that is capable of providing a high voltage. Suitable power sources that have a high voltage and a high energy density include lithium batteries, lithium ion secondary batteries, and double layer capacitors. Typically, these power sources comprise non-aqueous electrolytes as ion conductors. Solid, liquid and gel non-aqueous electrolytes are known.
Non-aqueous electrolytes, as referred to herein, are compositions that typically include an electrolyte carrier, e.g., a non-aqueous solvent, and an electrolyte salt solute, e.g., support salt, dissolved in the carrier. The electrolyte composition may be a solid solution as well as a liquid solution. For example, a non-aqueous electrolyte may be formed by dissolving an electrolyte support salt in an aprotic organic solvent, e.g., an ester, ether, amide, sulfalone etc. Well known solvents used for this purpose include cyclic carbonic acid esters, acyclic carbonic acid esters, cyclic carboxylic acid esters such as dimethyl carbonate, diethyl carbonate, propylene carbonate, dimethoxyethane, and mixtures thereof. Common electrolyte salts include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium tetrachloroaluminate (LiAlF4), lithium bromide (LiBr), and lithium hexaftuoroarsenate (LiAsF6).
Despite advances in the art, there are performance and safety issues still encountered with electrolytes used in batteries and the like. For example, improvements are still sought in charge and discharge characteristics of such devices, e.g., prolonging the charge and discharge cycle life of a non-aqueous-electrolyte secondary battery having a large discharging capacity. Also, aprotic solvents, such as those found in liquid and gel electrolytes, or used in the preparation of solid electrolytes, are usually combustible materials which may increase the risk of fires, especially when solvents leak from the respective device. An attempt to improve one characteristic of an electrolyte must not come at the expense of other characteristics, for example, improvements in safety must not adversely affect charge and discharge characteristics, and vice versa.
For example, some attempts to improve flame retardance have adversely affected battery performance. Addition of a phosphate such as trimethyl phosphate or the like to a non-aqueous electrolyte as disclosed in JP-A-H4-184870, JP-A-H8-22839 and JP-A-2000-182669 will improve flame retardance. However, these phosphates decompose during storage under high voltage, or during discharge and recharge, limiting battery performance. Similarly, a battery using a non-aqueous electrolyte comprising a combination of a fluorinated phosphate and a phosphazene compound, as disclosed in JP-A-2006-107910, exhibits high non-combustibility and good initial battery performances, but the battery capacity unacceptably decreases over time under exposure to high temperatures in a charged state, or when discharge-recharge are repeated at a low voltage.
Nitrogen containing compounds such as amines, amine salts and other amine derivatives have been used in non-aqueous electrolytes to improve performance and enhance safety related characteristics. For example, U.S. Pat. No. 5,759,714, incorporated herein by reference, discloses a non-aqueous electrolyte secondary battery with excellent charge/discharge characteristics and longer lasting discharge capacity obtained by using an electrolyte containing a nitrogen containing organic compound, such as an amine or an alkali metal salt of an aromatic amine.
Nitrogen containing compounds have also been used to facilitate improvements in flame retardance. U.S. Patent Application Publication No. 2008/0020285, incorporated herein by reference, discloses a flame retardant non-aqueous electrolyte for a battery comprising a cyclic phosphazene compound, a difluorophosphate compound, an aniline derivative and a support salt. The electrolyte and battery of U.S. Pat Appl Pub No. 2008/0020285 is similar to those of JP-A-H4-184870, JP-A-H8-22839, JP-A-2000-182669 and JP-A-2006-107910 above, but it appears that the presence of the aniline derivative helps prevent degradation of the flame retardant phosphorus compounds while improving the high temperature and long term performance of the battery.
It has been found that certain polycyclic aromatic amines, such as naphthyl amines, are surprisingly more suitable than other amine additives for use in non-aqueous electrolytes intended for use in batteries, capacitors and the like. The present naphthyl amines are effective stabilizers for components of the non-aqueous electrolyte and enhance the charge/discharge characteristics of a device containing it. The amines of the present electrolyte are less prone to degradation themselves during, e.g., repetition of discharge/recharge cycles, than the nitrogen containing compounds exemplified in U.S. Pat. No. 5,759,714 or described in U.S. 200810020285, and exhibit high temperature stability. Also, the present amines, and electrolytes containing them, do not foul electrodes, which is known to greatly reduce efficiency of a battery.